1. Field of the Invention
The present invention relates to a semiconductor memory device, e.g., a semiconductor memory device including a memory element capable of storing a binary digit by supplying currents bidirectionally.
2. Description of the Related Art
Recently, many solid-state memories for storing data on the basis of new principles have been proposed. In particular, a magnetic random access memory (MRAM) using the tunneling magnetoresistive effect is attracting attention as a solid-state magnetic memory. The MRAM is characterized by storing data in accordance with the magnetization state of a magnetic tunnel junction (MTJ) element.
In the conventional MRAM in which data is written by a magnetic field generated by a line current, the holding force increases when the size of the MTJ element decreases. This often increases the write current. The conventional MRAM described above cannot decrease the cell size for the purpose of increasing the capacity and decrease the current at the same time.
A spin-injection-type MRAM using the spin momentum transfer (SMT) write method as a write method capable of solving the above problem has been proposed. In this spin-injection-type MRAM, data is written by a current flowing through the MTJ element in a direction perpendicular to the film surface, and the spin direction in a free layer is changed by the direction of this current. Also, a memory cell comprises an MTJ element and selection transistor. For example, this selection transistor shares a source region with an adjacent cell in order to reduce the cell area.
When two selection transistors share the source region in the spin-injection-type MRAM, the source potential is supplied by a wiring layer by (1) forming a source potential line parallel to the gate electrode of the selection transistor, or (2) forming a source potential line perpendicularly to the gate electrode of the selection transistor. Method 2 has the problem that in an active area formed perpendicularly to the gate electrode of the selection transistor, no contact can be directly formed from the source potential line to the source region of the selection transistor.
As a related technique of this kind, a technique that improves the reliability and suppresses crosstalk by increasing the degree of integration in an MRAM in which data is written by using a current magnetic field is disclosed (Jpn. Pat. Appln. KOKAI Publication No. 2003-218326).